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Biology Coursework - Osmosis

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Investigation Planning. What are you planning to investigate? I am planning to investigate how solutions of different molarities of sodium chloride affect a potatoes tissue in size and mass and to explain why and how this happens. Variable Factors, which may affect investigation: > Length > Area > Shape > Mass > Temperature Which variables are you going to change? I hope to use three variables, independent, dependant and fixed variables. These should help me to keep the experiment a fair test, see each result properly and be able to thoroughly explain the results. My independent variable will be the solutions. I am going to use 6 different solutions of different molarities in order to see how results change depending on it. I will use solutions of 0M, 0.1M, 0.2M, 0.4M, 0.6M, 0.8M and 1M or sodium chloride solution. I will have two dependant variables; these will be the length and mass of the potatoes. I will measure the length of each piece of potato before they are placed in the solutions and I will weigh them to get the total of the three pieces in each dish. After a period of time I will then come back and take the same results before comparing the two sets of data. My fixed variables will be the initial length of the pieces of potato, the time they are in the solution and the amount of solution in each dish. I will use the same potato for all the pieces as some potatoes may have different water levels to start with; I will use the same size borer for each piece and will cut them to the same length. Hopefully these variables will help me to keep the experiment constant and fair giving me reliable results which will help me to draw accurate conclusions from the investigation. What do you predict will happen in your experiment? ...read more.


I expect the first few concentrations to have a small change in mass, but then I expect a steep decrease in mass change percentage at around 0.2/0.4 M. I then expect the reaction rate, and overall change to slow down and decrease at a less steep gradient on the graph, as it approaches 0.6/0.8 M. This is because the osmosis will occur on different levels, depending on the concentration in the water. The water potential of pure water is 0, and as I add solute it will decrease. Water diffuses (osmosis) from where there is a high water potential, to where there is a low water potential. The potato will have a lower water potential than pure water, so at first the water, will diffuse into the potato. However, as I add calcium carbonate the water potential outside the potato will decrease so water will start to diffuse from the potato into the surrounding solution. Initial Sizes and Weights of the Potato Tissue and the Solutions. The initial pieces of potato I cut were cut out of a normal potato using a size 5 borer. I then cut the tissue up into tubes 3cm long. I took great care whilst doing this, to ensure that the pieces were all the same length and that the experiment would be fair. I used the same borer and ruler for each piece too. In each petri dish I decided to put in 3 pieces of potato tissue. I then weighed each piece individually. I used different amounts of water and sodium chloride in each solution in order to get accurate solutions and measurements. To make sure the solutions were accurate I used a measuring cylinder to measure the water and sodium chloride. Molar Solution (M) Amount of Water (ml) Amount of sodium chloride (ml) 0.0 50 0 0.2 40 10 0.4 30 20 0.6 20 30 0.8 10 40 1.0 0 50 The pieces of potato tissue were all put into the petri dishes as 3:15pm on the 25th May. ...read more.


I used a ruler to cut the pieces to 3cm, however because a ruler is only accurate to around 0.5mm, some of the lengths may not have been accurate enough. There are a couple of ways I could improve on the accuracy of my pieces, firstly I could have used a top pan balance, to gain more accurate measurements, or, a better and more reliable way, would have been to use a laser to measure each piece so that they were all equal. In order to find out how reliable my results were I decided to do a percentage uncertainty on my results. I found that my percentages were quite interesting. For some reason I had a higher percentage uncertainty in the weaker concentrations, and a very low percentage in the high concentrations. The only explanation I have for this is that the levels, which the pieces decreased by in the higher concentrations, were closer together than that of the weaker concentrations, therefore the mean change was lower and the entire equation for the percentage uncertainty was lower. If I were to repeat this investigation I think that I would spend more time, using more reliable devices, such as a laser, to attain more accurate measurements and therefore results with lower percentage uncertainties. This would improve the overall outcome of my investigation. I could also do the experiment with different concentrations, and more of them i.e. 0.1M, 0.15M, 0.2M etc. This would provide a wider variety of results, which would make my estimate of the isotonic point, and concentration inside the potato, more accurate. Overall I am pleased with how my investigation went. I collected the results I needed, could draw conclusions from them and could relate them to my predictions and the scientific reasoning behind them. There are improvements I could have made, which I am fully aware of, however as far as this experiment went I am satisfied with the outcome. ...read more.

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Here's what a teacher thought of this essay

3 star(s)

*** A well carried out experiment in the laboratory with a good deal of detailed scientific knowledge in the report
Planning The report needed more care in the planning stage where there is a lot of confusion over the chemical being used. It needs to be much clearer in the identification of the key variables and how each is controlled. A good prediction of the expected results backed up the right amount of theory.
Obtaining Evidence The experiment seems to have been carried out well with a good range and a reasonable number of replicates. Headings on tables need more detail.
Analysing results It would have been good to see the plotted graph. A pattern was identified and explained well.
Evaluation This needed a few more suggestions as to how the method could be improved or extended. Only a few sources of possible errors have been identified.

Marked by teacher Stevie Fleming 03/01/2013

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